Ground freezing has been used to stabilize weak/loose soil, shore excavations and cut off groundwater since the 1800's when it was first used in Europe. Since then it has been applied to many hundreds of projects where other types of ground improvement or shoring were not effective. Conventional uses of ground freezing include, but are not limited to: vertical frozen soil shoring around an excavation to temporarily cut off groundwater and retain soil, horizontal freezing outside a shaft to stabilize the ground during tunnel boring machine (TBM) mining, horizontal freezing through a railroad or highway embankments to stabilize ground during pipe jacking, deep circular frozen soil shoring for construction of a TBM access shafts.
Typically, ground freezing employs a freeze plant, such as a chiller, connected to manifold lines that in turn feed into a series of vertical freeze pipes, installed around the perimeter of a site. Conventional freeze pipes consist of welded steel pipe ranging from 3 to 6 inches in diameter. As best shown in FIG. 1, a center pipe 12 of smaller diameter is disposed inside the outer steel pipe 14 to allow chilled brine to flow down from the top of the freeze pipe 10, along the outside annulus of the center pipe 12, and circulate evenly throughout its entire length. This type of conventional freeze pipe is termed “Full Freeze Pipe (FFP), since it freezes the soil from the ground surface to the bottom of the pipe. The brine then flows back up through the smaller center pipe 12. A steel head 16 fits on the top of the freeze pipe 10 to separate the inflowing and outflowing brine. Brine, usually a calcium chloride solution, is then pumped through the chillers and the freeze pipes at temperatures that can ultimately reach −20° F. or colder.
Frozen soil begins to occur immediately around each freeze pipe when chilled brine is circulated through the system. Since brine is circulated evenly throughout the entire length of the freeze pipe, the frozen soil formation extends from the ground surface to the tip of the freeze pipe. The frozen soil usually forms as a cylinder around each pipe, and can extend to well over 4 feet in radius over time. For the most common application, vertical shoring around a site or shaft, freeze pipes are usually spaced at 3 to 5 feet apart so that over time, a solid wall of frozen soil forms around the site from the ground surface to the bottom of the freeze pipe. Frozen soil is essentially impermeable and very strong (some frozen soil can be nearly ½ the strength of concrete), resulting in a very effective technique for groundwater cutoff and shoring during excavation.